CN107023355B

CN107023355B - Exhaust gas purification system and control method thereof

Info

Publication number: CN107023355B
Application number: CN201611051668.4A
Authority: CN
Inventors: 郑镇宇
Original assignee: Hyundai Motor Co
Current assignee: Hyundai Motor Co
Priority date: 2015-12-11
Filing date: 2016-11-24
Publication date: 2020-05-19
Anticipated expiration: 2036-11-24
Also published as: US20170167337A1; US10161281B2; CN107023355A

Abstract

The present disclosure provides an exhaust gas purification system, comprising: an engine; a lean NOx trap installed on the exhaust pipe and enabling absorption of nitrogen oxides contained in the exhaust gas at a lean air/fuel ratio or release of the absorbed NOx at a rich air/fuel ratio; a selective catalytic reduction catalyst disposed downstream of the LNT to reduce NOx contained in the exhaust gas; a controller performing denitrification by using the LNT and/or the SCR catalyst based on driving conditions of the engine; a first oxygen sensor disposed between the engine and the LNT to detect an amount of oxygen in the exhaust gas; a second oxygen sensor disposed between the LNT and the SCR catalyst to detect an amount of oxygen in exhaust gas discharged from the LNT; and an air injection device selectively injecting air into the exhaust pipe.

Description

Exhaust gas purification system and control method thereof

Cross Reference of Related Applications

This application claims priority and benefit from korean patent application No. 10-2015-0177109, filed on 2015, 12/11, which is incorporated herein by reference in its entirety.

Technical Field

The present disclosure relates to an exhaust gas purification system and a control method thereof.

Background

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

Generally, exhaust gas flowing out of an engine through an exhaust manifold is driven into a catalytic converter installed at an exhaust pipe and purified therein. Then, the noise of the exhaust gas is reduced while passing through the muffler and then the exhaust gas is discharged to the atmosphere through the tail pipe. The catalytic converter purifies pollutants contained in exhaust gas. Further, a particulate filter for trapping Particulate Matter (PM) contained in the exhaust gas is installed in the exhaust pipe.

A denitrification (DeNOx) catalyst is used in one type of such catalytic converter, and purifies nitrogen oxides (NOx) contained in exhaust gas. If a reducing agent such as urea, ammonia, carbon monoxide, and Hydrocarbon (HC) is supplied to the exhaust gas, NOx contained in the exhaust gas is reduced by an oxidation-reduction reaction of the DeNOx catalyst with the reducing agent.

Recently, a Lean NOx Trap (LNT) catalyst has been used as such a DeNOx catalyst. The LNT catalyst absorbs NOx contained in the exhaust gas when the air/fuel ratio is lean and releases the absorbed NOx when the air/fuel ratio is rich, and reduces the released NOx and NOx contained in the exhaust gas when the air/fuel ratio is rich (hereinafter referred to as "regeneration of LNT").

However, since a typical diesel engine operates with a lean air/fuel ratio, it requires the air/fuel ratio to be artificially adjusted to rich in order to release adsorbed NOx from the LNT. For this purpose, the time to release the absorbed NOx in the LNT (i.e., the regeneration time) should be accurately determined.

In addition, if the temperature of the exhaust gas is high (for example, the temperature of the exhaust gas is higher than 400 ℃), the LNT cannot remove NOx contained in the exhaust gas. To solve such a problem, a Selective Catalytic Reduction (SCR) catalyst is used together with the LNT.

Here, when the air/fuel ratio of the engine is rich, the content of the non-combusted fuel contained in the exhaust gas and passing through the LNT increases. We have therefore found that the oxygen concentration must be controlled to activate the redox reaction of a Selective Catalytic Reduction (SCR) catalyst.

The above information disclosed in this background section is only for enhancement of understanding of the background of the disclosure and, therefore, the disclosure may contain information that does not constitute prior art that is already known to a person of ordinary skill in the art.

Disclosure of Invention

The present disclosure provides an exhaust gas purification system and a control method thereof, which are advantageous in that nitrogen oxides contained in exhaust gas are removed by activating an oxidation-reduction reaction of a Selective Catalytic Reduction (SCR) catalyst by controlling the concentration of oxygen contained in the exhaust gas upstream of the SCR catalyst when the air/fuel ratio of an engine is rich.

An exhaust gas purification system according to an exemplary form of the present disclosure may include: an engine including an injector configured to inject fuel, the engine being configured to generate power by combusting a mixture of air and fuel, and to discharge exhaust gas generated by the combustion to the outside through an exhaust pipe; a Lean NOx Trap (LNT) mounted on the exhaust pipe and configured to absorb nitrogen oxides (NOx) contained in the exhaust gas at a lean air/fuel ratio, release the absorbed NOx at a rich air/fuel ratio, and reduce the nitrogen oxides contained in the exhaust gas or the released nitrogen oxides using a reducing agent containing carbon or hydrogen contained in the exhaust gas; a Selective Catalytic Reduction (SCR) catalyst installed at an exhaust pipe downstream of the LNT and configured to reduce NOx contained in exhaust gas by passing through the LNT; a controller configured to perform denitrification (DeNOx) by using at least one of the LNT and the SCR catalyst based on driving conditions of the engine; a first oxygen sensor mounted on the exhaust pipe and disposed between the engine and the LNT, the first oxygen sensor configured to detect an amount of oxygen in exhaust gas discharged from the engine; a second oxygen sensor mounted on the exhaust pipe and disposed between the LNT and the SCR catalyst, the second oxygen sensor being configured to detect an amount of oxygen in exhaust gas discharged from the LNT; and an air injection device configured to selectively inject air into an interior of the exhaust pipe based on a control signal of the controller, the air injection device being disposed between the second oxygen sensor and the SCR catalyst.

The controller may operate the LNT such that the regeneration of NOx is performed based on an amount of oxygen contained in the exhaust gas detected by the first oxygen sensor.

The controller may operate the air injection device to selectively inject air into the exhaust gas based on an amount of oxygen in the exhaust gas detected by the second oxygen sensor.

The third oxygen sensor may be mounted on the exhaust pipe and disposed between the air injection device and the SCR catalyst, and the third oxygen sensor is configured to detect an amount of oxygen contained in exhaust gas containing air discharged from the air injection device and send a signal corresponding to the amount of oxygen to the controller.

The controller may control the air injection device so as to control the air injection amount based on an oxygen amount in the exhaust gas detected by a third oxygen sensor upstream of the SCR catalyst.

A control method of an exhaust gas purification system provided with an LNT and a Selective Catalytic Reduction (SCR) catalyst according to an exemplary form of the present disclosure may include: driving the vehicle in an air injection drive mode; identifying regeneration of the entering NOx at the LNT when the lambda value upstream of the LNT is less than 1; when the lambda value downstream of the LNT is less than 1, controlling an air injection amount upstream of the SCR catalyst by operating the air injection device, and determining whether the lambda value downstream of the LNT is greater than 1.

The lambda value upstream of the LNT is measured by a first oxygen sensor mounted on the exhaust pipe upstream of the LNT.

The lambda value downstream of the LNT is measured by a second oxygen sensor mounted on the exhaust pipe downstream of the LNT.

The control method of the exhaust gas purification system may further include: it is determined whether the lambda value upstream of the SCR catalyst is equal to a predetermined value after controlling the air injection amount by operating the air injection device.

The lambda value upstream of the SCR catalyst can be measured by a third oxygen sensor mounted on the exhaust pipe upstream of the SCR catalyst.

The predetermined value may be approximately between 1.002 and 1.457.

The present disclosure is directed to providing an exhaust gas purification system and a control method thereof such that an oxidation-reduction reaction of a Selective Catalytic Reduction (SCR) catalyst is activated by controlling a concentration of oxygen contained in exhaust gas upstream of the SCR catalyst when an air/fuel ratio of an engine is rich to thereby improve purification efficiency of NOx.

Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

Drawings

In order that the disclosure may be fully understood, various aspects of the disclosure will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of an exhaust gas purification system according to one form of the present disclosure;

FIG. 2 is a flow chart of a control method of an exhaust purification system according to one form of the present disclosure;

FIG. 3 is a schematic illustration of an exhaust gas purification system according to another form of the present disclosure; and

fig. 4 is a flowchart of a control method of an exhaust gas purification system according to another form of the present disclosure.

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

Description of the symbols

10: engine

20: exhaust pipe

30: lean NOx Trap (LNT)

40: selective Catalytic Reduction (SCR) catalyst

50: controller

60: air injection device

Detailed Description

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

First, since the exemplary forms described in the specification and the configurations shown in the drawings are only exemplary forms and configurations of the present disclosure, they do not represent all the technical concepts of the present disclosure, and it should be understood that various equivalents and modifications that may substitute for the exemplary forms are possible when the present application is filed.

For clarity of description of the present disclosure, portions irrelevant to the description are omitted, and the same or similar constituent elements are denoted by the same reference numerals throughout the present disclosure.

Since the size and thickness of each configuration shown in the drawings are arbitrarily illustrated for convenience of description, the present disclosure is not necessarily limited to the configurations shown in the drawings, and the thicknesses are illustrated exaggerated for clarity of illustrating several parts and regions.

Furthermore, throughout this disclosure, unless explicitly described to the contrary, the word "comprise", and variations such as "comprises" or "comprising", will be understood to imply the inclusion of stated elements but not the exclusion of any other elements.

Furthermore, terms such as ". directional unit", ". directional device", ". directional component", and ". directional member" described in this disclosure refer to a fully constructed unit having at least one function or operation.

FIG. 1 is a schematic illustration of an exhaust gas purification system according to an exemplary form of the present disclosure.

As shown in fig. 1, an exhaust system of an internal combustion engine includes an engine 10, an exhaust pipe 20, a Lean NOx Trap (LNT)30, a Selective Catalytic Reduction (SCR) catalyst 40, and a controller 50.

Engine 10 combusts an air/fuel mixture of fuel and air to convert chemical energy into mechanical energy. The engine 10 is connected to an intake manifold 16 to receive air in the combustion chambers 12, and to an exhaust manifold 18 so that exhaust gas generated during combustion is collected in the exhaust manifold 18 and discharged to the outside. The injector 14 is installed in the combustion chamber 12 to inject fuel into the combustion chamber 12.

Diesel engines are illustrated herein, but lean-burn gasoline engines may be used. In the case of a gasoline engine, an air/fuel mixture flows into the combustion chamber 12 through the intake manifold 16, and a spark plug (not shown) is mounted on an upper portion of the combustion chamber 12. Further, if a Gasoline Direct Injection (GDI) engine is used, the injector 14 is installed at an upper portion of the combustion chamber 12.

An exhaust pipe 20 is connected to the exhaust manifold 18 to discharge exhaust gas to the outside of the vehicle. The LNT 30 and the SCR catalyst 40 are mounted on the exhaust pipe 20 to remove hydrocarbons, carbon monoxide, particulate matter, and nitrogen oxides (NOx) contained in the exhaust gas.

A first oxygen sensor 32 is mounted downstream on the exhaust pipe 20 between the engine 10 and the LNT 30. The first oxygen sensor 32 detects the amount of oxygen in the exhaust gas discharged from the engine 10, and sends a signal corresponding thereto to the controller 50 in order to assist the controller 50 in performing lean/rich control of the exhaust gas. In the present disclosure, the value detected by the first oxygen sensor 32 is referred to as a lambda value (oxygen value) upstream of the LNT.

The LNT 30 is mounted on the exhaust pipe 20 downstream of the engine 10. The LNT 30 absorbs nitrogen oxide (NOx) contained in the exhaust gas at a lean air/fuel ratio and releases the absorbed nitrogen oxide, and reduces nitrogen oxide contained in the exhaust gas or released nitrogen oxide at a rich air/fuel ratio. In addition, the LNT 30 may oxidize carbon monoxide (CO) and Hydrocarbons (HC) contained in the exhaust gas.

Herein, the hydrocarbon means all compounds consisting of carbon and hydrogen contained in the exhaust gas and the fuel.

A second oxygen sensor 34 is mounted on the exhaust 20 downstream of the LNT 30. The second oxygen sensor 34 detects an oxygen amount contained in the exhaust gas flowing into the SCR catalyst 40 and sends a signal corresponding thereto to the controller 50.

The controller 50 may perform lean/rich control of the exhaust gas based on the values detected by the first oxygen sensor 32 and the second oxygen sensor 34. In the present disclosure, the value detected by the second oxygen sensor 34 is referred to as the lambda value downstream of the LNT.

The SCR catalyst 40 is installed on the exhaust pipe 20 downstream of the LNT 30, and reduces nitrogen oxides contained in the exhaust gas by using reducing agents containing carbon, hydrogen, nitrogen, or oxygen and generated or not oxidized from the LNT 30.

The controller 50 determines a driving condition of the engine, and performs lean/rich control and controls the amount of reducing agent injected by a dosing module (dosing module) based on the driving condition of the engine. For example, the controller 50 may release NOx from the LNT 30 by controlling the air/fuel ratio to be rich and may reduce the released NOx by using a reductant contained in the exhaust gas (this will be referred to as "regeneration of the LNT" in the present disclosure). In addition, the controller 50 may remove NOx at the SCR catalyst 40 by using reductant that is produced or not oxidized from the LNT 30. The lean/rich control may be performed by controlling the amount of fuel injected by the injector 14.

Here, the controller 50 operates the LNT 30 so that NOx regeneration (i.e., regeneration of the LNT) is performed according to the amount of oxygen contained in the exhaust gas by a signal detected by the first oxygen sensor 32.

When the air injection device 60 receives a control signal from the controller 50, it may selectively inject air into the interior of the exhaust pipe 20 between the second oxygen sensor 34 and the SCR catalyst 40.

Accordingly, the controller 50 selectively operates the air injection device 60 to inject air into the exhaust gas prior to flowing into the SCR catalyst 40 when the lambda downstream of the LNT is below a predetermined value based on the amount of oxygen in the exhaust gas read from the signal sent from the second oxygen sensor 34.

The plurality of maps, characteristics of the LNT, and/or correction factors enable controller 50 to determine a regeneration start time and a regeneration end time. Multiple mappings, LNT characteristics, and/or correction factors may be set through multiple experiments.

The controller 50 may be in the form of one or more processors that are activated by a predetermined program, and the predetermined program may be written to perform each step of the method of regenerating the LNT.

The first oxygen sensor 32 and the second oxygen sensor 34 are electrically connected to the controller 50, and send the detected values to the controller 50.

The first oxygen sensor 32 detects the amount of oxygen in the exhaust gas and sends a signal corresponding thereto to the controller 50. The controller 50 may perform lean/rich control of the exhaust gas based on the amount of oxygen in the exhaust gas detected by the first oxygen sensor 32. The value detected by the first oxygen sensor 32 may be expressed as a lambda (λ) value. The lambda value represents the ratio of the actual air/fuel ratio to the stoichiometric air/fuel ratio. If the lambda value is greater than 1, the air/fuel ratio is lean, whereas if the lambda value is less than 1, the air/fuel ratio is rich.

The second oxygen sensor 34 detects an amount of oxygen in the exhaust gas emitted from the LNT 30 and sends a signal corresponding thereto to the controller 50.

A plurality of sensors other than the sensor shown in fig. 1 may be installed in the exhaust gas purification apparatus. However, for better understanding and ease of description, the description of the plurality of other sensors will be omitted.

Hereinafter, referring to fig. 2, a control method of an exhaust gas purification system according to an exemplary form of the present disclosure will be described in detail.

Fig. 2 is a flowchart of a control method of an exhaust gas purification system according to an exemplary form of the present disclosure.

As shown in fig. 2, the control method of the exhaust gas purification system according to the exemplary form of the present disclosure is executed while the vehicle is running. If the vehicle is started, the controller 50 controls the engine 10 to operate in the air injection mode in step S1.

The controller 50 receives an input of the amount of oxygen in the exhaust gas from the first oxygen sensor 32 and determines whether the lambda value upstream of the LNT is less than 1 in step S2.

When it is determined in step S2 that the lambda value upstream of the LNT is less than 1, the controller 50 identifies entry into NOx regeneration at the LNT 30 in step S3.

That is, the LNT 30 releases the adsorbed NOx and performs LNT regeneration, which reduces the released nitrogen oxides and nitrogen oxides contained in the exhaust gas.

In contrast, when the controller 50 determines that the lambda value upstream of the LNT is not less than 1 (i.e., when the condition is not satisfied), the controller 50 returns to step S1 of driving the vehicle in the air injection driving mode.

In step S4, the controller 50 receives an input of the amount of oxygen in the exhaust gas passing through the LNT 30 from the second oxygen sensor 34 and determines whether the lambda value downstream of the LNT is less than 1.

When it is determined that the lambda value downstream of the LNT is less than 1, the controller 50 controls the air injection amount upstream of the SCR catalyst 40 by operating the air injection device 60 in step S5.

Therefore, the amount of oxygen in the exhaust gas increases before flowing into the SCR catalyst 40, and the oxidation-reduction reaction may be activated in the SCR catalyst 40.

Conversely, when the controller 50 determines that the lambda value downstream of the LNT is greater than 1 (i.e., when the condition is not satisfied), the controller 50 returns to step S3 identifying entry into NOx regeneration at the LNT 30.

The controller 50 then receives an input of the amount of oxygen in the exhaust gas passing through the LNT 30 from the second oxygen sensor 34 and determines whether the lambda value downstream of the LNT is greater than 1 in step S6.

When it is determined in step S6 that the lambda value downstream of the LNT is greater than 1, the controller 50 stops actuating the air injection device 60 and ends the control in step S7.

In contrast, when the controller 50 determines that the lambda value downstream of the LNT is less than 1 (i.e., when the condition is not satisfied), the controller 50 returns to step S5 of controlling the air injection amount by operating the air injection device 60.

Specifically, the control method of the exhaust gas purification system according to the present disclosure has an advantage of removing nitrogen oxides contained in the exhaust gas by activating the oxidation-reduction reaction in the SCR catalyst 40 by controlling the oxygen concentration contained in the exhaust gas when the air/fuel ratio of the engine is rich.

FIG. 3 is a schematic illustration of an exhaust gas purification system according to another exemplary form of the present disclosure.

As shown in fig. 3, an exhaust system of an internal combustion engine includes: an engine 10, an exhaust pipe 20, a Lean NOx Trap (LNT)30, a Selective Catalytic Reduction (SCR) catalyst 40, and a controller 50.

As an exemplary form, a diesel engine is described, but a lean-burn gasoline engine can be used in another form. In the case of a gasoline engine, an air/fuel mixture flows into the combustion chamber 12 through the intake manifold 16, and a spark plug (not shown) is mounted on an upper portion of the combustion chamber 12. Further, if a Gasoline Direct Injection (GDI) engine is used, the injector 14 is installed at an upper portion of the combustion chamber 12.

An exhaust pipe 20 is connected to the exhaust manifold 18 to discharge exhaust gas to the outside of the vehicle. The LNT 30 and the SCR catalyst 40 are mounted on the exhaust pipe 20 to remove hydrocarbons, carbon monoxide, particulate matter, and nitrogen oxides (NOx) contained in the exhaust gas. A first oxygen sensor 32 is mounted downstream on the exhaust pipe 20 between the engine 10 and the LNT 30. The first oxygen sensor 32 detects the amount of oxygen in the exhaust gas discharged from the engine 10, and sends a signal corresponding to the amount of oxygen to the controller 50 in order to assist the controller 50 in performing lean/rich control of the exhaust gas. In the present disclosure, the value detected by the first oxygen sensor 32 is referred to as the lambda value upstream of the LNT.

Herein, the hydrocarbon means all compounds composed of carbon and hydrogen contained in the exhaust gas and the fuel.

A second oxygen sensor 34 is mounted on the exhaust 20 downstream of the LNT 30. The second oxygen sensor 34 detects an oxygen amount contained in the exhaust gas flowing into the SCR catalyst 40 and sends a signal corresponding to the oxygen amount to the controller 50.

The controller 50 determines a driving condition of the engine, and performs lean/rich control and controls an amount of the reducing agent injected by the dosing module based on the driving condition of the engine. For example, the controller 50 may release NOx from the LNT 30 by controlling the air/fuel ratio to be rich and may reduce the released NOx by using a reductant contained in the exhaust gas (this will be referred to as "regeneration of the LNT" in the present disclosure). In addition, the controller 50 may remove NOx at the SCR catalyst 40 by using reductant that is produced or not oxidized from the LNT 30. The lean/rich control may be performed by controlling the amount of fuel injected by the injector 14.

Here, the controller 50 operates the LNT 30 so as to perform NOx regeneration (i.e., regeneration of the LNT) according to the amount of oxygen contained in the exhaust gas by a signal detected from the first oxygen sensor 32.

Here, the third oxygen sensor 42 may be mounted on the exhaust pipe 20 arranged between the air injection device 60 and the SCR catalyst 40. The third oxygen sensor 42 detects the amount of oxygen contained in the exhaust gas containing air from the air injection device 60.

Specifically, the third oxygen sensor 42 detects an oxygen amount contained in the exhaust gas upstream of the SCR catalyst 40 and sends a signal corresponding to the oxygen amount to the controller 50.

The controller 50 controls the air injection device 60 so as to control the air injection amount based on the oxygen amount in the exhaust gas detected from the third oxygen sensor 43 upstream of the SCR catalyst 40.

The controller 50 may perform lean/rich control of the exhaust gas based on the values detected by the first oxygen sensor 32, the second oxygen sensor 34, and the third oxygen sensor 42. In the present disclosure, the value detected by the third oxygen sensor 42 is referred to as the lambda value upstream of the SCR catalyst.

The controller 50 is provided with a plurality of maps, characteristics of the LNT, and correction coefficients, and thus it can determine the regeneration start time and the regeneration end time based on them. Multiple maps, characteristics of the LNT, and/or correction factors may be set through multiple experiments.

The controller 50 may be in the form of one or more processors that are activated by a predetermined program, and the predetermined program may be written to perform each step of a method of regenerating an LNT according to an exemplary form of the present disclosure.

The first oxygen sensor 32, the second oxygen sensor 34, and the third oxygen sensor 42 are electrically connected to the controller 50, and send the detected values to the controller 50.

The first oxygen sensor 32 detects the amount of oxygen in the exhaust gas and sends a signal corresponding to the amount to the controller 50. The controller 50 may perform lean/rich control of the exhaust gas based on the amount of oxygen in the exhaust gas detected by the first oxygen sensor 32. The value detected by the first oxygen sensor 32 may be expressed as a lambda (λ) value. The lambda value refers to the ratio of the actual air/fuel ratio to the stoichiometric air/fuel ratio. If the lambda is greater than 1, the air/fuel ratio is said to be "lean", and if the lambda is less than 1, the air/fuel ratio is said to be "rich".

The second oxygen sensor 34 detects an amount of oxygen in the exhaust gas emitted from the LNT 30 and sends a signal corresponding to the amount of oxygen to the controller 50. The third oxygen sensor 42 detects the amount of oxygen contained in the exhaust gas containing the air injected by the air injection device 60 and sends a signal corresponding to the amount of oxygen to the controller 50.

A plurality of sensors other than the sensor shown in fig. 3 may be installed in the exhaust gas purification apparatus in other forms. However, for better understanding and ease of description, the description of the plurality of other sensors will be omitted.

Hereinafter, referring to fig. 4, a control method of an exhaust gas purification system according to another form of the present disclosure will be described in detail.

Fig. 4 is a flowchart of a control method of the exhaust gas purification system.

As shown in fig. 4, the control method of the exhaust gas purification system is executed when the vehicle is driven. If the vehicle is started, the controller 50 controls the engine 10 to operate in the air injection mode in step S10.

The controller 50 receives an input of the amount of oxygen in the exhaust gas discharged from the engine 10 from the first oxygen sensor 32, and determines whether the lambda value upstream of the LNT is less than 1 in step S20.

When it is determined in step S20 that the lambda value upstream of the LNT is less than 1, the controller 50 identifies entry into NOx regeneration at the LNT 30 in step S30.

More specifically, the LNT 30 releases the adsorbed NOx, and performs LNT regeneration, which reduces the released nitrogen oxides and nitrogen oxides contained in the exhaust gas.

In contrast, when the controller 50 determines that the lambda value upstream of the LNT is not less than 1 (i.e., when the condition is not satisfied), the controller 50 returns to step S10 of driving the vehicle in the air injection driving mode.

The controller 50 then receives an input of the amount of oxygen in the exhaust gas passing through the LNT 30 from the second oxygen sensor 34 and determines whether the lambda value downstream of the LNT is less than 1 in step S40.

When it is determined that the lambda value downstream of the LNT is less than 1 in step S40, the controller 50 controls the air injection amount upstream of the SCR catalyst 40 by operating the air injection device 60 in step S50.

In contrast, when the controller 50 determines that the lambda value downstream of the LNT is not less than 1 (i.e., when the condition is not satisfied), the controller 50 returns to step S30 identifying entry into NOx regeneration at the LNT 30.

In another form, after controlling the amount of air injection by operating the air injection device 60 in step 50, the controller 50 determines whether the lambda value upstream of the SCR catalyst is equal to a predetermined value in step 60.

Here, the lambda value upstream of the SCR catalyst is detected by a third oxygen sensor 42 mounted on the exhaust pipe 20 upstream of the SCR catalyst 40. The third oxygen sensor 42 detects the amount of oxygen contained in the exhaust gas containing air from the air injection device 60 and sends a signal corresponding to the amount of oxygen to the controller 50.

The predetermined value may be in the range from about 1.002 to 1.457 and in one form the predetermined value may be about 1.236.

Specifically, when it is determined in step S60 that the lambda value upstream of the SCR catalyst is equal to the predetermined value, the controller 50 receives an input of the amount of oxygen in the exhaust gas passing through the LNT 30 from the second oxygen sensor 34, and determines in step S70 whether the lambda value downstream of the LNT is greater than 1.

Meanwhile, when the controller 50 determines that the lambda value upstream of the SCR catalyst is not the predetermined value (i.e., when the condition is not satisfied), the controller 50 returns to step S50 of controlling the air injection amount by operating the air injection device 60.

When it is determined in step S70 that the lambda value downstream of the LNT is greater than 1, the controller 50 stops actuating the air injection device 60 and ends the control in step S80.

In contrast, when the controller 50 determines that the lambda value downstream of the LNT is not greater than 1 (i.e., when the condition is not satisfied), the controller 50 returns to step S50 of controlling the air injection amount by operating the air injection device 60.

That is, the method of the exhaust gas purification system according to another exemplary form may repeat the above process when the air/fuel ratio of the engine is rich, and has an advantage of removing nitrogen oxides contained in the exhaust gas by activating the oxidation-reduction reaction in the SCR catalyst 40 by controlling the oxygen concentration contained in the exhaust gas.

As described above, when the air/fuel ratio of the engine is rich, the purification efficiency of NOx can be improved by controlling the oxygen concentration contained in the exhaust gas upstream of the Selective Catalytic Reduction (SCR) catalyst so that the oxidation-reduction reaction of the SCR catalyst can be activated.

While the disclosure has been described in connection with what is presently considered to be practical exemplary forms, it is to be understood that the disclosure is not limited to the disclosed forms, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the disclosure.

Claims

1. An exhaust gas purification system comprising:

an engine including an injector configured to inject fuel, the engine being configured to generate power by combusting a mixture of air and fuel and to discharge exhaust gas generated by the combustion through an exhaust pipe;

a lean-burn nitrogen oxide trap installed on the exhaust pipe and configured to absorb nitrogen oxide contained in the exhaust gas at a lean air/fuel ratio, release the absorbed nitrogen oxide at a rich air/fuel ratio, and reduce the nitrogen oxide contained in the exhaust gas or the released nitrogen oxide using a reducing agent including carbon or hydrogen contained in the exhaust gas;

a selective catalytic reduction catalyst installed at the exhaust pipe downstream of the lean nitrogen oxide trap and configured to reduce nitrogen oxides contained in exhaust gas by passing through the lean nitrogen oxide trap;

a controller configured to perform denitrification by using at least one of the lean nitrogen oxide trap and the selective catalytic reduction catalyst based on a driving condition of the engine;

a first oxygen sensor mounted on the exhaust pipe and disposed between the engine and the lean-burn nitrogen oxide trap, the first oxygen sensor configured to detect an amount of oxygen in an exhaust gas emitted from the engine;

a second oxygen sensor mounted on the exhaust pipe and disposed between the lean-burn nitrogen oxide trap and the selective catalytic reduction catalyst, the second oxygen sensor configured to detect an amount of oxygen in exhaust gas discharged from the lean-burn nitrogen oxide trap; and

an air injection device configured to selectively inject air into an interior of the exhaust pipe based on a control signal of the controller, the air injection device being disposed between the second oxygen sensor and the selective catalytic reduction catalyst; and

a third oxygen sensor mounted on the exhaust pipe and disposed between the air injection device and the selective catalytic reduction catalyst, and configured to detect an amount of oxygen contained in exhaust gas containing air discharged from the air injection device and send a signal corresponding to the amount of oxygen to the controller.

2. The exhaust gas purification system according to claim 1, wherein

The controller is configured to operate the lean-burn nitrogen oxide trap such that regeneration of the nitrogen oxides is performed based on an amount of oxygen contained in the exhaust gas detected by the first oxygen sensor.

3. The exhaust gas purification system according to claim 1, wherein

The controller is configured to operate the air injection device to selectively inject air into the exhaust gas based on an amount of oxygen in the exhaust gas detected by the second oxygen sensor.

4. The exhaust gas purification system according to claim 1, wherein

The controller is configured to control the air injection device so as to control an air injection amount based on an oxygen amount in the exhaust gas detected by the third oxygen sensor upstream of the selective catalytic reduction catalyst.

5. A control method of an exhaust gas purification system provided with a lean-burn nitrogen oxide trap and a selective catalytic reduction catalyst, comprising:

driving the vehicle in an air injection drive mode;

identifying regeneration of nitrogen oxides entering at the lean nitrogen oxide trap when a lambda value upstream of the lean nitrogen oxide trap is less than 1;

controlling an air injection quantity upstream of the selective-catalytic-reduction catalyst by operating an air injection device when a lambda value downstream of the lean-burn nitrogen oxide trap is less than 1, and

determining whether a lambda value downstream of the lean nox trap is greater than 1;

determining whether a lambda value upstream of the selective catalytic reduction catalyst is equal to a predetermined value after controlling the air injection amount by operating the air injection device,

wherein the lambda value upstream of the selective-catalytic-reduction catalyst is measured by a third oxygen sensor mounted on the exhaust pipe upstream of the selective-catalytic-reduction catalyst,

wherein the third oxygen sensor is arranged between the air injection device and the selective catalytic reduction catalyst, and the third oxygen sensor is configured to detect an amount of oxygen contained in exhaust gas containing air discharged from the air injection device.

6. The control method according to claim 5, wherein:

the lambda value upstream of the lean nox trap is measured by a first oxygen sensor mounted on the exhaust pipe upstream of the lean nox trap.

7. The control method according to claim 5, wherein

The lambda value downstream of the lean nox trap is measured by a second oxygen sensor mounted on the exhaust pipe downstream of the lean nox trap.

8. The control method according to claim 5, wherein

The predetermined value is between 1.002 and 1.457.